The invention relates to a circuit arrangement for signaling in telecommunications networks, as claimed in the preamble of patent claim 1, and as is known from U.S. Pat. No. 4,190,745.
Private branch exchanges (PBX), digital multichannel transmission systems (DAML=Digital Added Main Line) or digital transmission systems (DLC=Digital Loop Carrier) have functions for signaling operating states. For this purpose resistances are connected between the individual transmission lines by, for example, a private branch exchange. A switching center, which is connected to the private branch exchange, then applies a voltage between one line and the other lines, which are short-circuited, and measures the resultant currents through the lines. The switching center can use the measured currents to deduce the resistances connected between the individual transmission lines. By varying the resistance values, for example, the private branch exchange can transmit information about operating states to the switching center. For example, in this case, it is possible to signal whether a voice transmission or a data transmission is intended to take place. Alternatively, the switching center can find out whether the transmission lines are faulty. In this case, however, different resistance values must be provided and the resistance values must be matched to specific national requirements for the transmission lines, particularly when using public telecommunications network transmission lines.
If there are three transmission lines, the measurement is carried out by using switches, which are preferably in the form of relays, to connect resistances between the transmission lines. Two transmission lines are in each case short-circuited to one another, as a result of which two resistances are in each case connected in parallel. The switching center applies a voltage to the remaining transmission lines. The resultant currents through the transmission lines and resistances are measured, and the resistance values are calculated from them. The calculation results are also referred to as a signature. A signature makes it possible, for example, to deduce the state of the transmission lines or a specific operating state. If one of the transmission lines is broken, then no current can flow via this line and the resistance value determined by the switching center in this measurement, or the signature, does not match the predetermined resistance value or the signature for an uninterrupted line. A disadvantage in this case is that the relays for connecting the resistances between the transmission lines are expensive. A correspondingly large number of resistances must be provided if there are a number of different signatures. Furthermore, a resistance must be provided for each transmission line, and these resistances must be matched to the specific national requirements. It is either necessary to construct different circuits for different resistance values, or parallel resistance paths must be provided, with a correspondingly large number of relays.
A circuit arrangement for detecting faults on a subscriber line is known from U.S. Pat. No. 4,710,949, which uses switches that are sensitive to voltage and current and are connected in the lines. A disadvantage in this case is, however, that these switches are highly complex and are thus expensive. Furthermore, these switches can influence the transmission of signals.
The invention is thus based on the technical problem of specifying a circuit arrangement for signaling in telecommunications networks which, firstly, can be matched to different requirements and, secondly, can be constructed with little circuitry complexity.
This object is achieved by a circuit arrangement for signaling in telecommunications networks having the features of patent claim 1. Advantageous refinements of the circuit arrangement can be found in the respective dependent claims.
A circuit arrangement for signaling in telecommunications networks has a first connection and a second connection for a first and a second conductor, respectively, in a transmission line. The first connection is connected to the first input of a rectifier, and the second connection is connected to the second input of said rectifier. The positive output and the negative output of the rectifier are connected to one another via a variable resistance. A third connection for a third conductor in the transmission line can be connected via the series circuit formed by a switch and a first diode to the positive output of the rectifier, and via the series circuit formed by the switch and a second diode to the negative output of the rectifier.
The control loop, the rectifier and the transistor and resistance are used for setting a line impedance during normal operation of a line circuit which, for example, links a digital subscriber circuit to an analog transmission line. Thus, advantageously, the invention provides a circuit arrangement for signaling in telecommunications networks with little additional circuitry complexityxe2x80x94two diodes and one switch. Instead of having to provide a specific resistance for testing each conductor in a transmission line, which is connected by means of a relay between two conductors in the transmission line for testing, already existing circuits are provided with additional circuits for testing the transmission line.
In one preferred embodiment, a control loop is provided to which a first voltage, which is produced at the positive output of the rectifier, and a second voltage, which is produced at the negative output of the rectifier, are supplied. The control loop produces a control voltage which controls the variable resistance. The transfer function of the control loop is variable. A particularly advantageous feature in this case is that the capability to vary the transfer function of the control loop makes it possible to vary the variable resistance, and thus the resistance between two conductors in the transmission line, during testing. The circuit arrangement can thus be matched to specific national requirements, and different signatures can be produced, without changing the circuitry.
It is particularly preferable for the variable resistance to have a transistor.
The cathode of the first diode is preferably connected to the positive output of the rectifier, and the anode of the second diode is preferably connected to the negative output of the rectifier.
The control loop preferably has a digital filter, with the transfer function of the control loop being variable by programming the filter coefficients of the digital filter. It is particularly preferable in this case for the digital filter to be formed by an appropriately programmed digital signal processor.
In one preferred embodiment, the control loop has an analog integrator circuit which is connected upstream of the transistor and integrates the difference between a first input voltage and a second input voltage, and whose output signal controls the transistor.
It is particularly preferable for the transistor to be an n-channel MOSFET.
The circuit arrangement for signaling in telecommunications networks is also suitable for being added to an integrated circuit, by virtue of the use of electronic components. In this case, of course, all the other circuits, such as the control loop, the rectifier and the transistor and resistors, can also be integrated at the same time.
Further advantages, features and application options for the invention will become evident from the following description of exemplary embodiments in conjunction with the drawing. In the drawing: